The invention relates to direct thermal-to-electric energy conversion systems for in situ recovery of underground geothermal energy.
High temperature geological areas, commonly referred to as geothermal areas, exist throughout the world. These areas are a natural source of heat that can be used to generate electricity. This can be done by drilling a hole or by making use of existing holes, both natural and artificial. There are a number of current systems, which utilize these areas to produce electricity, but there are several problems associated with them.
One system used is hydroelectric power—geothermal heat is used to heat up water to high temperatures, which produces steam to power turbines. This involves either using natural hot springs or pumping water down holes, bringing the water to the surface and setting up a generating system above ground. There are a lot of inefficiencies involved in this system as heat is lost along the way. It is costly to build and maintain, as there are many moving parts, all of which are subject to wear and tear. Furthermore, the circulating water (or other fluid) often dissolves large quantities of minerals and becomes very corrosive. The dissolved minerals also often precipitate out as the fluid escapes or is pumped from underground. The precipitating minerals can effectively plug the well. All of these systems also require that the fluid be vaporized at some point.
Prior art patents such as U.S. Pat. No. 6,301,894 to Halff and U.S. Pat. No. 4,189,923 to Berg have taken steps to avoid the problems associated with mineral buildup. Other systems use dry steam—steam from geothermal areas is used directly to power turbines. These methods avoid the problems that arise when water is used, but still necessitate the building of a generating system and the cost and time that this involves.
Systems have been invented to utilize the heat energy to produce electricity in a downhole environment, thus avoiding the need to bring heat to the surface, through liquid or any other medium, and avoiding the problems encountered with water as mentioned above. These systems use devices that can produce electricity by the simple application of heat to the device. The electricity produced in this way needs only to be conducted to the surface, where it can be used instantly. One prior art method of doing this involves thermoelectric devices. U.S. Pat. No. 6,150,601 to Schnatzmeyer et al., for example, refers to the use of thermoelectric devices to generate electricity in a downhole environment, both for the benefit of recharging battery packs and to generate an independent electricity supply. It should be noted that the electricity produced is “DC”, so for “AC” applications a converter must me used, also, because of the nature of the device, some form of voltage control must be used. The transformer can be placed underground or above ground depending on if it if more beneficial to have it easily accessible or to boost the voltage near the source to increase transport efficiency.
This avoids many of the problems of previous systems, but has considerable drawbacks. Thermoelectric devices generate power by using special materials and configurations that force heat to push electrons from one side of the device to the other.
The biggest problem with thermoelectrics is that while heat pushes electrons in one direction, the material itself redistributes most of that heat through simple conduction. This means that most of the heat is not usefully harnessed, and instead flows through the system in all directions, reducing efficiency. Prior art patent U.S. Pat. No. 4,356,401 to Santi describes a thermo-electric power station supplied by a geothermal heat source. This system still requires liquid to be heated underground, necessitating the building of a geothermal system and a power station. Furthermore, the power station may have the problems associated with thermo-electrics mentioned above.
Similar ideas have been developed using thermionic systems as opposed to thermo-electric systems. These have the advantage of being far more efficient because there is a physical gap between two substrates, which prevents heat from returning to its source. However, prior art inventions involving thermionic systems are only able to function efficiently at very high temperatures, thus limiting the areas in which they can be used. Furthermore, prior art systems include expensive custom designed units fully encircling centralized heat pipes, such as patent U.S. Pat. No. 4,047,093 to Levoy.
Patent Application number WO99/13562 of Borealis Technical Limited describes a method for generating electricity from any heat source using thermotunneling converters. These are diode devices made by placing two materials very close to each other so that energetic electrons can tunnel from one material to the next. By tapping this electron flow, usable electric current can be extracted. The gap between the two materials ensures that the temperature differential between the two sides is maintained. It also allows current to flow in one direction only. Being that the most energetic electrons tunnel, heat is likely to be transferred from the hot side to the cold side along with the current. The gap acts as a heat sink and prevents the heat from being transferred by mere conduction.
This system has been shown to be extremely efficient and has the added advantage of being able to harness lower grade heat than both turbine systems and the annular prior art thermionic systems mentioned above. It also has the potential require no external mechanical or electrical power, thus allowing it to be completely self-sufficient.